Devices are known in the prior art to fold and interlock foldable sections of a flat carton blank to form an erected carton suitable as a container for numerous materials. One broad class of carton erecting machines is the vertical die and plunger type. In general, a reciprocal plunger moving through an "up and down" cycle in a vertical plane forces a flat carton blank through a forming die open at both top and bottom. The sides of the die whether stationary or movable in whole or in part, fold the side and end panels of the blank around the moving plunger as the plunger passes downwardly through the die, thereby erecting the side and end panels relative to the bottom of the box.
Today, where usable for the ultimate containing function, a glueless, interlocking carton will be preferred. Foldable corner lock flaps, each having a tab insertable into a cooperating slot located in an adjacent erected panel, are folded by the cooperating die and plunger, with the last erecting function of the apparatus usually being the locking of the "tab" into its cooperating slot. This is usually accomplished by movable parts on the die and/or plunger which push or pull the tab through the insertion slot. With erection complete, the carton is stripped off the plunger as it changes direction and passes upwardly, back through the die. The carton will then be forwarded to the next step in an integrated packaging operation.
Such a carton erecting machine is described in detail in my prior U.S. Pat. No. 3,859,896 assigned to the same assignee as the present invention. In that carton erecting machine, knife edges, pivotally mounted on the plunger and spring-biased outwardly therefrom, press against the side walls of the carton adjacent the slots, thereby opening the slots to receive their respective tabs. The tabs are each inserted behind the knife edges, each tab having an upper portion which extends above the upper edge of its slot on the outside of the carton. When all of the tabs have been so inserted, the knife edges are caused to move inwardly thereby pulling the tabs, including the upper extending portions thereof, through the side panel slots to form an interlocking engagement. The assembled carton is then removed from the plunger as the latter is withdrawn from the die.
One problem encountered with such a device, is that the inward pulling of the knife edges causes the assembled carton to be rather securely held so that the force required to strip the carton from the plunger is significantly increased.
Furthermore, since the assembled carton is held very closely against the plunger, engagement of the carton by the carton stripper becomes difficult. In order to assure effective engagement of the carton, it is preferable to maintain the carton strippers very close to the lateral surfaces of the plunger but such an arrangement may interfere with the formation of the carton during the downward motion of the plunger. Therefore, it is necessary to use a moving carton stripper which is retracted during the plunger's downward motion and is extended inwardly during the withdrawal of the plunger.